1. Field of the Invention
The present subject matter relates to software defined radio, and more particularly to a radio capable of operation with multiple protocols.
2. Background
Two significant forms of radio communication are referred to as AIS and ORBCOMM.
AIS or Automatic Identification System is a short range coastal tracking system used on ships and by Vessel Traffic Services (VTS) for identifying and locating vessels by electronically exchanging identification, position, course, and speed data with other nearby ships and VTS stations. Class A AIS radio is required under an international convention to be fitted aboard international voyaging ships with gross tonnage (GT) of 300 or more tons, and all passenger ships regardless of size. Class A AIS radios are significantly more expensive than other classes of AIS radios and have a robust set of capabilities.
Other classes of AIS radios are Class B and receive (RX) only. These are simpler, and are used in fishing vessels and leisure craft. They have a smaller set of capabilities. A Class B transponder provides both AIS reception and transmission at a fraction of the cost of a conventional class A transponder. The Class B AIS radio requires use of Digital Selective Calling (DSC) channel 70 for channel management. Additionally, a GPS antenna is included. The RX only AIS radio does not include a calling channel.
In order to prevent overloading of available bandwidth, Class B transmissions are restricted to 2 watts. This limits range for vessels using Class B AIS to a range of about 5 to 10 miles. At the present time, almost all Class B units use software defined radio. The transmitted signal is a standard AIS data stream at 9600 bps using Gaussian Minimum Shift Keying (GMSK). Generally, an AIS radio is equipped with a serial interface acceptingRS-232 and/or NMEA formats.
ORBCOMM satellites are low Earth orbit communications satellites, operated by the United States satellite communications company Orbcomm, Inc. As of 2008, 44 such satellites were in orbit. The ORBCOMM Satellite Communication System is a wide area, packet switched, two-way data communication system that utilizes constellations of the ORBCOMM satellites and earth station gateways. These satellites relay digitized data in the vicinities of 137 MHz and 150 MHz. A terrestrial ORBCOMM radio communicates with a satellite. A nominal ORBCOMM radio may comprise a single board microprocessor based VHF transceiver capable of transmitting and receiving messages in cooperation with the Orbcomm Satellite Communication System. ORBCOMM customers access the gateway and thus, the satellite, via dial up circuits, the Internet, or X.25 protocol access systems. The ORBCOMM radios transmit between 148.00 and 150.05 MHz at 5 to 10 watts using 2400 bps Symmetric Differential Phase Shift Keying (SDPSK) modulation and receive downlink 4800 bps SDPSK modulated signals between 137.0 and 138.0 MHz. They access the satellite via an ORBCOMM proprietary acquire-communicate TDMA/FDMA protocol.
The AIS and ORBCOMM radios operate on diverse frequencies and use different forms of signal modulation. The data structure for packets in each system is different. If a ship or other communications platform wishes to use both AIS and ORBCOMM communications, the operator must buy separate AIS and ORBCOMM radios.